一个简单的sql 语句

求助一个简单的sql 语句
SELECT ZYH, HCRQ, ZZRQ
FROM ZY_HCMX
WHERE (HCLX = 0) AND (HQCH IS NULL) AND (HCRQ >= '2012-1-1') AND (HCRQ < '2012-2-1')
ORDER BY ZYH


结果：
218162012-01-01 8:01:38NULL
218182012-01-01 10:25:37NULL
218192012-01-01 9:11:52NULL
218202012-01-01 10:07:57NULL
218212012-01-01 10:15:24NULL
218212012-01-20 15:57:53NULL


我想要的结果是（每一个 住院号，只取日期小的一个数据）
218162012-01-01 8:01:38NULL
218182012-01-01 10:25:37NULL
218192012-01-01 9:11:52NULL
218202012-01-01 10:07:57NULL
218212012-01-01 10:15:24NULL

如何处理？

[解决办法]

SQL code

SELECT ZYH, HCRQ, ZZRQFROM ZY_HCMX tWHERE (HCLX = 0) AND (HQCH IS NULL) AND (HCRQ >= '2012-1-1') AND (HCRQ < '2012-2-1')    and not exists (select 1 from ZY_HCMX WHERE (HCLX = 0) AND (HQCH IS NULL) AND (HCRQ >= '2012-1-1') AND (HCRQ < '2012-2-1')                    and ZYH=t.ZYH and HCRQ<t.HCRQ)ORDER BY ZYH
[解决办法]
SQL code
SELECT ZYH, (select min(HCRQ)from ZY_HCMX WHERE ZYH=z.ZYH  )HCRQ, ZZRQFROM ZY_HCMX zWHERE (HCLX = 0) AND (HQCH IS NULL) AND (HCRQ >= '2012-1-1') AND (HCRQ < '2012-2-1')ORDER BY ZYH
[解决办法]
顶一下吧，坐等楼下解释